When removing user that has content associated with his/her username, you will obtain following output listing the space name where that user has contributed by adding content.
Searching for that particular piece of content is not that easy.
My suggestion is: When fetching the content associated with a username, make it possible to conveniently access that content via links or give a specific Space > page or Page > Comment links to get to it.

 
The following error(s) occurred:

    * This user can not be removed

« Return to view user: xyz@xyz.com

XYZ can not be removed because he or she has authored content in one or more spaces.
The user has commented in the following spaces:

    * Confluence Extension